1. Field of the Invention
This invention relates to the field of current monitors, and particularly to “high side” current monitors.
2. Description of the Related Art
A high side current monitor is designed to measure the signal current through a sensing element connected in series with a circuit's high side (as opposed to its return side). A shunt voltage proportional to the signal current is developed across the sensing element—typically a small resistor. The current monitor measures the differential voltage across the sensing element, and produces a ground or common-referred output that varies with the sensed current.
A conventional high side current monitor is shown in FIG. 1. A sensing element 10, here a resistor having a resistance Rs, is connected in series with a signal 12 having a voltage V1 and carries a current of interest Isense to a load 14; Rs is typically on the order of 0.1 Ω. An operational amplifier A1 is connected across the sensing element, with its inverting input connected to the sensing element's load side, and its non-inverting input connected to the sensing element's high side via a resistor 15 having a resistance R1. A feedback transistor Q0, here a NPN, has its base connected to the output of A1, its collector connected to the junction of R1 and A1's non-inverting input, and its emitter providing an output Iout. Iout is delivered to an output resistor 16 having a resistance Rout to produce an output voltage Vout.
In operation, Isense develops a shunt voltage Vsense across Rs; A1 responds by causing Q0 to conduct a current through R1 necessary to equalize A1's inverting and non-inverting inputs. This current (Iout) is proportional to the voltage (Vsense) across—and thus to the current (Isense) through—sensing element 10. As output voltage Vout=IoutRout, it is also proportional to current of interest Isense.
When the current is sensed on the high side of a circuit (as in FIG. 1), the differential voltage applied to A1 can have a large common mode potential. An op amp IC has an associated breakdown voltage determined by its fabrication process, which limits its common mode input range—which in turn limits the signals with which the current monitor of FIG. 1 can be safely used.